I have flushed the auto recently and put some anti slip oil additive in the transmission. Trans was slipping from 3rd to 4th.
The car was shuddering before the transmission flush. Apart from the shudder the car runs well. Maybe a little rough while idle at the lights. But not rough enough to worry about.

would say that possibly one or more bands/clutch packs in the auto have had their day, or a valve or two in the auto has a little too much play, letting pressure/flow past it and reducing the force on the clutch packs, or the torque converter has issues.

also could be the old head gasket issue as taximan said...

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also, just check that theres no coolant thats leaked on the coilpack from the heater hoses/valve. I had this when i had an EF 6 and the shuddering on take off felt exactly like it was the auto when infact it wasnt.